Enhanced Efficiency of MAPbI₃ Perovskite Solar Cells with FAPbX₃ Perovskite Quantum Dots

Abstract

We describe a method to enhance power conversion efficiency (PCE) of MAPbI₃ perovskite solar cell by inserting a FAPbX₃ perovskite quantum dots (QD-FAPbX₃) layer. The MAPbI₃ and QD-FAPbX₃ layers were prepared using a simple, rapid spin-coating method in a nitrogen-filled glove box. The solar cell structure consists of ITO/PEDOT:PSS/MAPbI₃/QD-FAPbX₃/C₆₀/Ag, where PEDOT:PSS, MAPbI₃, QD-FAPbX₃, and C₆₀ were used as the hole transport layer, light-absorbing layer, absorption enhance layer, and electron transport layer, respectively. The MAPbI₃/QD-FAPbX₃ solar cells exhibit a PCE of 7.59%, an open circuit voltage (Voc) of 0.9 V, a short-circuit current density (Jsc) of 17.4 mA/cm², and a fill factor (FF) of 48.6%, respectively.

1. Introduction

Hybrid organic-inorganic perovskite materials have been widely accepted and applied in solar cells. The power conversion efficiency has evolved from 3.8% in 2009 to a certified value of 22.7% in 2018 [1,2,3,4,5]. Perovskite materials are a versatile material with unique optoelectronic properties, exhibiting strong light absorption, long diffusion length, and high mobility [6]. Developing high quality perovskite films is critical to improving perovskite solar cell efficiency. It is well known that high-performance solar cells originate from a low absorption bandgap (the optimal absorption band gap around a value of 1.1–1.5 eV) upon solar spectrum [7,8,9], low exciton binding energy [10,11], and long carrier diffusion length [12]. In addition, perovskite solar cells offer additional characteristics, like thin-film, flexibility, semitransparency, lightweight, and low-costs processing. A significant number of investigations have been focused on perovskite quantum dots (QDs) due to relatively abundant perovskite component sources [13,14,15]. The perovskite QD material present several advantageous properties, including band engineering through size and composition control, high absorption coefficient, light-response ranges (light absorption over a wide range of wavelengths from UV-visible to near IR), multiple exciton generation, cost-effectiveness, and solution process ability. Consequently, they are regarded as good light harvesters in perovskite solar cells [16,17,18,19,20]. While perovskite solar cells have experienced a steep increase in performance efficiency, they also show great potential to become a low-cost alternative to conventional solar cells. Due to the limited bandgap tunability of the perovskite material, near-infrared photons cannot be effectively captured in standard perovskite absorber layers by perovskite solar cells. We report on a two-step spin-coating perovskite solar cell manufacturing process, in which the MAPbI₃/QD-FAPbX₃ perovskite architecture is formed while using solvent-engineering techniques. Besides, the MAPbI₃/QD-FAPbX₃ layer exhibits a marginally broad light absorption region; the current density is also prominently enhanced, which is beneficial for improving the power conversion efficiency. These results explain why perovskite solar cells containing FAPbX₃ QDs exhibit a much higher PCE than cells without.

2. Materials and Methods

The 0.2 m mol of FAPbX₃ (X = I, Br) (FAPbBr_1.5I_1.5 and FAPbBrI₂) perovskite QD solution were prepared in this study while using a simple and rapid method.

Table 1. Parameters of FAPbX₃ (X = I, Br) perovskite quantum dots (QDs) solution.

Material	FAPbBr1.5I1.5	FAPbBrI2
FAI	17.1 mg	--
FAB	12.5 mg	25 mg
PbI2	46.1 mg	92.2 mg
PbBr2	36.7 mg	--

shows the FAI (Lumtec Corp., Taipei, Taiwan), FAB (Lumtec Corp., Taipei, Taiwan), PbI₂ (Alfa Aesar, Lancashire, UK), and PbBr₂ (Alfa Aesar, Lancashire, UK) perovskite FAPbX₃ (X = I, Br) QDs solution parameters. The FAB powder, FAI powder, PBI₂ powder, and PbBr₂ powder were decanted into a solution of 2 mL of dimethylformamide (DMF) (Echo Chemical Co., Ltd., Miaoli, Taiwan); and, 300 µL of oleic acid (Echo Chemical Co., Ltd., Miaoli, Taiwan) and 5 µL of n-octylamine (Echo Chemical Co., Ltd., Miaoli, Taiwan) were decanted into this mixture to form the FAPbX₃ (X = I, Br) precursor solution. Subsequently, A 20 µL of FAPbX₃ (X = I, Br) precursor solution was decanted into 2 mL of chlorobenzene (Echo Chemical Co., Ltd., Miaoli, Taiwan) and 3 mL of ethyl acetate (Echo Chemical Co., Ltd., Miaoli, Taiwan), followed by the centrifugal process to separate the red precipitate from the FAPbX₃ (X = I, Br) precursor solution. The said red precipitate was dried under vacuum for 12 h to entirely eliminate the solvent. The FAPbX₃ (X = I, Br) powder was then dissolved in 60 µL of chlorobenzene to prepare the FAPbX₃ (X = I, Br) perovskite QD solution.

The patterned ITO glass (Ruilong Corp., Miaoli, Taiwan) was cleaned using ultrasonic treatment in deionized (DI) water, acetone, and isopropanol. The ITO glass was then treated in a UV cleaner for 10 min. The cleaned ITO glass was coated with a PEDOT:PSS solution (UMAT Corp., Hsinchu, Taiwan) using a spin-coating method at 4000 rpm for 30 s, followed by heating at 120 °C for 10 min. The solution of the perovskite precursor was prepared using the 289 mg Pbl₂ and 98 mg methylammonium iodide (MAI) (Lumtec Corp., Taipei, Taiwan) solvent in 500 µL of a dimethyl sulfoxide (DMSO) (Echo Chemical Co., Ltd., Miaoli, Taiwan) and DMF cosolvent with a volume ratio of 1:9 in a glove box that was filled with forming gas of nitrogen. Next, the solution of the perovskite precursor were spin-coated onto the PEDOT:PSS/ITO glass using two-step spin-coating processes with 1000 rpm and 5000 rpm for 10 s and 20 s, respectively. At the second coating step, the said spin-coated layer was suppressed by the dropping 100 µL of anhydrous toluene (Echo Chemical Co., Ltd., Miaoli, Taiwan). Subsequently, the spin-coating process was undertaken and the samples were annealed at temperature 100 °C for 5 min. Afterward, the FAPbX₃ (X = I, Br) (FAPbBr_1.5I_1.5 and FAPbBrI₂) QDs dispersed in chlorobenzene (60 µL) were spin coated onto the above MAPbI₃ perovskite layer at 1000 rpm for 30 s, followed by standing for 2 min, respectively. Finally, a C₆₀ layer (Uni-Onward Corp., New Taipei, Taiwan) and Ag electrode (UMAT Corp., Hsinchu, Taiwan) were thermally formed sequentially by a thermal evaporator method with a high-vacuum ambient of 1.5 × 10⁻⁶ torr to finish the device structure. The samples were shielded with a shadow mask during the C₆₀/Ag forming, which define an active area of 0.5 × 0.2 cm². Figure 1 schematically describes the complete structure. The PEDOT:PSS, MAPbI₃, QD-FAPbX₃, and C₆₀ roles in the device structure are the hole transport layer, light-absorbing layer, absorption enhancement layer, and electron transport layer, respectively

3. Results and Discussion

Figure 2 shows the TEM (transmission electron microscopy) (Tecnai F30, Philips, Netherlands) images of the QD-FAPbBr_1.5I_1.5 and QD-FAPbBrI₂ thin films. It is clear that the QD-FAPbBr_1.5I_1.5 and QD-FAPbBrI₂ thin films are composed of many quantum dots, in which the quantum dot sizes ranged between 7 nm and 10 nm.

Figure 3 presents the photoluminescence (PL) spectra of the QD-FAPbBr_1.5I_1.5 and QD-FAPbBrI₂ solution were obtained using a photoluminescence system (UniRAM, Protrustech). The QD-FAPbBr_1.5I_1.5 and QD-FAPbBrI₂ peak locations are observed at 680 nm and 730 nm, respectively. This indicated that the band gap of QD-FAPbX₃ can be expected to decrease when the proportion of PbI₂ increases, such that it results in a red shift in the emission peak of the PL spectrum. That is, the observed red shift of the PL emission can be interpreted as a lowering of the band gap of the QD-FAPbX with a PbI₂ proportion increase. Insets are pictures of the QD-FAPbBr_1.5I_1.5 and QD-FAPbBrI₂ specimens that emitted near infrared light while being excited by a 405-nm laser.

Figure 4 plots the transmittance and absorbance spectra of MAPbI₃, MAPbI₃/QD-FAPbBr_1.5I_1.5, and MAPbI₃/QD-FAPbBrI₂ films measured using an UV/VIS/NIR spectrophotometer (U-4100, Hitachi). MAPbI₃, MAPbI₃/QD-FAPbBr_1.5I_1.5, and MAPbI₃/QD-FAPbBrI₂ films transmittances (see Figure 4a) were below 10% between 300 nm and 700 nm, but above 30% between 750 nm and 850 nm. In particular, the highest value of the transmittance spectrum was above 50% between 780 nm and 850 nm in the MAPbI₃ film spectrum on ITO glass. The absorption edge location was at 750 nm, which corresponded to the MAPbI₃ film absorption. Figure 4b shows the absorption spectra of the MAPbI₃, MAPbI₃/QD-FAPbBr_1.5I_1.5, and MAPbI₃/QD-FAPbBrI₂ films. The absorption spectra between 300 nm and 850 nm show that the MAPbI₃/QD-FAPbBrI₂ film was higher than that of MAPbI₃ and MAPbI₃/QD-FAPbBr_1.5I_1.5 films. One may observe that the MAPbI₃/QD-FAPbBr_1.5I_1.5 and MAPbI₃/QD-FAPbBrI₂ films were absorbed with noteworthy between 780 nm and 850 nm, as compared to the MAPbI₃ films. This can increase the spectral absorption of the solar cell, thereby generating more short-circuit current density and enhancing the power conversion efficiency.

The electrical J-V (current density-voltage) properties were obtained using a Keithley 2420 sourcemeter under irradiation by a 100 W xenon lamp. The irradiation condition on the sample surface was adjusted at a power density of 100 W/m² (AM1.5). Figure 5 presents the relationship between the current density and the voltage (J-V) for MAPbI₃, MAPbI₃/QD-FAPbBr_1.5I_1.5, and MAPbI₃/QD-FAPbBrI₂ solar cells.

Table 2. Solar cell photovoltaic parameters. The measurement was carried out under simulated one sun (100 mW/cm²).

Sample	Voc (V)	Jsc (mA/cm2)	FF (%)	Eff (%)
MAPbI3	0.9	11	53	4.27
MAPbI3/QD-FAPbBr1.5I1.5	0.89	17	43.3	6.54
MAPbI3/QD-FAPbBrI2	0.9	17.4	48.6	7.59

summaries the performance of the three different solar cells. The MAPbI₃/QD-FAPbBrI₂ optimal device demonstrated outstanding performance: power conversion efficiency (Eff) = 7.59%, short-circuit current density (Jsc) = 17.4 mA/cm², open-circuit voltage (Voc) = 0.9 V, and fill factor (FF) = 48.6%. Clearly, the solar cell with the MAPbI₃/QD-FAPbBrI₂ perovskite absorber exhibited the greatest increase in absorption, ranging from 300 nm to 850 nm. The cells with the MAPbI₃/QD-FAPbBrI₂ and MAPbI₃/QD-FAPbBr_1.5I_1.5 perovskite absorbers exhibited the improved external quantum efficiency (EQE), as shown in Figure 5b. The EQE curves were used to calculate the integration current of cells based on MAPbI₃/QD-FAPbBrI₂, MAPbI₃/QD-FAPbBr_1.5I_1.5, and MAPbI₃ were 16.2, 15.8, and 7.9 mA/cm². These values are different from the Jsc values that were obtained from J-V curves, which may be due to the samples, has been stored in a N₂--filled glove box over half a year. This means that the FAPbX₃ perovskite quantum dots coated on MAPbI₃ can increase the short-circuit current. This means that the MAPbI₃/QD-FAPbBrI₂ perovskite absorber can produce a high-efficiency thin film solar cell.

5. Conclusions

In conclusion, we demonstrated PCE enhancement of the MAPbI₃ perovskite solar cells with FAPbX₃ perovskite quantum dot via increasing the absorption range of the spectral. The PL spectra of the QD-FAPbX₃ indicated a red shift when the proportion of PBI₂ increases due to the band gap decrease. The absorption spectra showed that the MAPbI₃/QD-FAPbX₃ films exhibited broad spectral absorption. The device performance was optimized while using a MAPbI₃/QD-FAPbBrI₂ layer, with a short-circuit current density (J_SC) = 17.4 mA/cm², open-circuit voltage (V_OC) = 0.9 V, fill factor (FF) = 48.6%, and power conversion efficiency (Eff) = 7.59%. The power conversion efficiency of the cell with the MAPbI₃/QD-FAPbX₃ film improved remarkably around 43.7% in the best case.